Field of the Invention
This invention relates generally to a method for measuring the length of a sealed air column, and more specifically, to a system and method that can be used to measure the depth to a subsurface water table when the only access to the aquifer is a slender tube.
Background Art
Hydrologists normally measure the depth to water in a water well using an electrical water level meter. Such electrical meters typically consist of a wire pair lowered into the well, and which produces an electrical signal when the pair contacts the water surface. Using depth markings on the wire pair, the depth to the water table is determined. Other less common devices use a powder coating on a tape which is wet by the water contact and, when withdrawn from the well, which tape can be measured for the depth to the water surface. These several methods depend upon the access to the well being sufficiently large in diameter to accept the device being lowered into the well. Water levels in tubing disposed down well boreholes are measured in a similar manner to measurements taken in a well bore, but again the tube diameter must allow the passage of the measurement device.
In very deep well bores, or very long tubing emplaced in wells, the cable or tape lowered into the well or tube can adhere to the wall of the well or tube due to wet film adhesion (surface tension of a wet film between the tape or cable and the tubing wall). Such adhesion can produce such a strong bond to the wall of the tube or well that the tape or cable cannot be withdrawn without tensile failure of the tape or cable. This adhesion is aggravated by a well or tube which is not perpendicular to the ground's surface (i.e. vertical), and where the measurement device lies against the wall of the tubular passage to the water level. This is often a problem for wells or tubing with more than 200 feet in depth to the water table.
Another device used to measure the water table in deep wells is a pressure transducer lowered to be located beneath the water table. The transducers used in this technique are relatively expensive.
Techniques used by this applicant, such as that disclosed in U.S. Pat. No. 8,424,377, require an initial or baseline water level measurement to normalize subsequent pressure measurement histories. The method and apparatus disclosed herein allow the depth to the water level to be measured in a very slender tube, with an inside diameter too small to allow the passage of the traditional measurement devices.